1. Field
The present invention relates to a lithographic apparatus and to a stage apparatus.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
U.S. Pat. No. 6,469,773 describes a stage apparatus for a lithographic apparatus. The stage apparatus may hold a substrate, which in the context of the document referred to here may includes a reticle or other patterning device. The stage apparatus as described includes a mechanism for generating a force that changes in accordance with an acceleration when the stage moves. The force is applied by the mechanism onto the substrate from the direction of the acceleration. Thereby, it is intended to compensate for a force between the stage and the substrate which is caused by the acceleration. The mechanism includes a lever which is pivotable about a pivot axis. One end of the lever to establish a contact with a side of the reticle, the other end of the lever being connected to a counter weight. When the stage is accelerated, the counter weight will, due to inertia, exert a force onto the lever which translates into a force by the lever onto the side of the reticle. As inertia effects are proportional to the acceleration of the stage, the force which is applied by the lever onto the substrate is also proportional to the acceleration.
The mechanism as described in U.S. Pat. No. 6,469,773 is intended to enable the stage to be subjected to high accelerations. This is because the higher the acceleration, the higher the forces that the substrate (such as a reticle) which is held by the stage, is subjected too. The substrate may be held by the stage making use of a variety of holding devices, such as a vacuum suction, a mechanic clamp, an electrostatic clamp, etc. In any of these situations, a risk of slippage of the substrate comes into existence at high accelerations, because the holding force with the substrate is held by the holding device may be insufficient to be able to withstand the high forces due to accelerations of the stage. A second aspect is that the high forces during acceleration may result in a (temporary) deformation of the substrate. Such deformation may result in an inaccuracy of the pattern which is to be applied onto the substrate. The mechanism as described in the US Patent as referred to here, is attended to counteract these effects: when the stage is accelerated, the lever will exert a force onto the side of the substrate, thereby intending to provide a force which to some extend compensates for the acceleration forces, thereby attempting to prevent slippage of the substrate (such as a reticle) and deformation thereof.
In the mechanism of U.S. Pat. No. 6,469,773, over-compensation or under-compensation may occur. The force which is to be applied by the lever onto the substrate is substantially proportional to the acceleration of the state and substantially proportional to a weight of the countermass. The force applied onto the substrate to be able to compensate for the acceleration however depends on a mass of the substrate (such as a reticle). Thus, the mechanism as described may only provide compensation for a substrate having a specific mass, for a substrate having a higher mass under-compensation is obtained, while for a substrate having a lower mass, over-compensation is obtained. Due to such under-compensation or over-compensation, again a deformation of the reticle at acceleration may take place. Also, a risk of slippage is again provided for as the force provided by the lever may too small or too large.
Furthermore, the counter masses will increase a total weight of the stage. This is of particular importance as accelerations and velocities of the stage are to be maximized in a lithographic apparatus design, to be able to achieve a fast processing. The additional mass introduced by the mechanism will increase a total weight of the stage, thereby decreasing a maximum acceleration to be achieved thereby. Again, a further problem may be found when applying an acceleration to the stage which is opposite to the direction of acceleration in which the mechanism would push against the reticle. In that case, the mechanism will come loose from the reticle. Now, when a direction of the acceleration changes, a collision of the mechanism against the reticle may occur which may damage the reticle and/or may statically or dynamically displace the reticle with respect to the stage, thereby creating a static or dynamic alignment or deformation error.
The lever of the mechanism described in U.S. Pat. No. 6,469,773 is fully pivotable about the pivot axis, being subjected to acceleration forces. In a state of constant velocity, this may imply that the lever rests against the side of the substrate, however it may also be the case that the lever is to a certain extent free from the substrate, e.g. leaving a gap therebetween. The inventors of the present patent applications have now devised that a source of inaccuracy comes forward therefrom. In case that the lever rests against the reticle, this appears to result in a small residual force onto the side of the substrate which in turn results in a small deformation of the substrate. This is of particular importance in a common lithographic apparatus, where irradiation takes place in the phase of constant velocity of the stage. The remaining force which results in a remaining deformation, thereby introduces an error in applying the pattern by the lithographic apparatus. Furthermore, this error is to a large extent unpredictable as the lever may or may not rest against the side of the substrate during illumination.